Fungicide efficacy and the impact of stripe rust on spring and winter wheat in South Africa.
Field trials were conducted from 1997 to 1999, in both the winter and summer rainfall areas of South Africa, to determine the effect of Puccinia striiformis f.sp. tritici [P. striiformis f. tritici] on the yield and quality of bread wheat (Triticum aestivum). During 1997 in the South Western Cape, combined seventh and flag leaf treatments of five triazole fungicides, over three cultivars, resulted in a 43% yield increase. Stripe rust head infections were most effectively controlled by the application of fungicides closely to or just after head emergence. The yield was negatively correlated with the area under the disease progress curve (AUDPC [R2=0.23]). In the absence of disease, during 1998, fungicide treatments failed to significantly increase yield or hectolitre mass. During 1999, the most effective control of stripe rust was obtained by applying propiconazole at both the seventh and flag leaf stages and where triticonazole seed treatment was applied in addition to the latter treatments. The latter two treatments resulted in a 49% yield increase. Yield was negatively correlated with the AUDPC (R2=0.85). Furthermore, significant differences were found among the fungicides carboxin/thiram, flutriafol/thiabendazole, tebuconazole, triadimenol, and triticonazole, applied as seed treatments on the cultivar Gariep. The AUDPC decreased by 61 and 25%, respectively, in plots where triadimenol and triticonazole were applied. The remaining three seed treatments were not significantly different from the control.